The present invention relates to a carbon heating element with a resistivity and shape that are required of a heating element, and to a method of producing it.
Worked articles of metal wire such as tungsten wire and Nichrome wire, machined articles of carbon such as isotropic carbon materials and glass carbon, and metal compounds such as silicon carbide have been the primary materials which are conventionally used for resistance heating elements. Among these substances, worked articles of metal wire have been mainly used as heating elements for heaters in small-sized commercial devices, while carbon and metal compounds have been used for industrial furnaces and the like.
Of the conventional materials used for heating elements, carbon differs from metal wire and the like in its excellent properties such as heating rate, heating efficiency and far infrared ray-generating efficiency. However, because conventional carbon heating elements are produced from large plate-like or block-like bodies by machining, the production process is complicated and costly, and production of thin rods and sheets is difficult. Moreover, such heating elements have a problem in that there is no option other than to vary the shape of the elements to control the calorific values of the elements because the heating elements are prepared by cutting blocks, etc., having resistivities in certain specified ranges.
In PCT/JP98/02849 there is proposed a carbon heating element obtained by mixing graphite powder and a metal or metalloid conduction-inhibiting substance such as boron nitride or silicon carbide, with a carbon-containing resin such as chlorinated vinyl chloride resin or the like, and then firing the mixture in an inert gas such as nitrogen gas.
This carbon heating element has excellent characteristics in that it allows control of the resistivity to any desired value by changing the proportion of the carbon as the good electric conductor and the metal or metalloid compound as the conduction-inhibiting substance, and in that it can be formed into any shape before firing to give the desired shape as a carbon heating element.
However, subsequent research has shown that impurity elements such as iron, calcium, potassium or sodium remain in the heating element depending on the starting substances, firing temperature, etc., and that this promotes aging upon repeated conduction use and thereby impairs the properties.
Specifically, when iron is included as an impurity at 100 ppm or greater in the heating element, black deposits are produced on the inner wall of the glass tube of the heater unit during its conduction use, which is undesirable as it prevents radiation of light and heat from the wire material.
When calcium is included at 100 ppm or greater, white deposits are produced on the inner wall of the glass tube of the heater unit during its conduction use, which is undesirable as it also prevents radiation of light and heat from the wire material,
In the presence of potassium and sodium, the crystal state of the SiO2 of the glass tubes of the heater unit is altered at the high temperature during conduction, becoming white and brittle. Embrittlement and loss of strength of the glass tube can result in leakage of oxigen gas in an environment and consumption of the heating element by oxidation. Radiation of light and heat is inhibited by embrittlement of the glass tubes.
It is therefore an object of the present invention to provide a carbon heating element with excellent aging resistance and thermal stability over years of repeated conduction use, as well as a method of producing it.
According to the invention, there is provided a carbon heating element comprising carbon as a good electrical conductor, and containing as impurity elements iron at a content of no greater than 100 ppm, calcium at a content of no greater than 100 ppm, potassium at a content of no greater than 50 ppm and sodium at a content of no greater than 50 ppm.
In this carbon heating element, the iron content is preferably no greater than 20 ppm, the calcium content is preferably no greater than 50 ppm and the potassium and sodium contents are preferably both no greater than 10 ppm.
The carbon heating element preferably also contains a metal or metalloid compound as a conduction-inhibiting substance.
The carbon heating element is produced by mixing a metal or metalloid compound with a composition exhibiting a carbon residual yield that is substantially not zero after firing, and then firing the mixture at a pressure of no higher than 1xc3x9710xe2x88x922 Pa and a temperature of from 1500xc2x0 C. to 2200xc2x0 C. Firing at high temperature in such a vacuum can remove the impurity elements that impair the aging resistance and thermal stability of the heating element.
Metals and metalloid compounds include commonly available metal carbides, metal borides, metal silicides, metal nitrides, metal oxides, metalloid nitrides, metalloid oxides, metalloid carbides and the like. The amount of metal or metalloid compound used may be appropriately selected depending on the resistance value and shape of the heating element to be produced, and any one or combination of two or more may be used, but for easier control of the resistance value it is particularly preferred to use boron carbide, silicon carbide, boron nitride or aluminum oxide, preferably at no greater than 80 parts by weight in order to also maintain the excellent properties of the carbon.
The aforementioned composition is an organic substance exhibiting a carbon yield of at least 5% upon firing in an inert gas atmosphere. Specifically there may be mentioned thermoplastic resins such as polyvinyl chloride, polyacrylonitrile, polyvinyl alcohol, polyvinyl chloride-polyvinyl acetate copolymer, polyamides, etc., thermosetting resins such as phenol resins, furan resins, epoxy resins, unsaturated polyester resins, polyimides, etc., natural polymer substances having condensed polycyclic aromatics in the basic structure of the molecule, such as lignin, cellulose, tragacanth gum, gum arabic, sugars, etc., as well as synthetic polymer substances having condensed polycyclic aromatics in the basic structure of the molecule, such as formalin condensate of naphthalenesulfonic acid, copna resins, and the like. The amount of the composition to be used may be appropriately selected depending on the shape of the heating element to be produced, and any one or combination of two or more may be used, but polyvinyl chloride resin and furan resin are particularly preferred; in order to ensure that the excellent properties of the carbon are maintained it is preferably used in an amount of 20 parts by weight or greater.
The composition preferably contains carbon powder. As carbon powders there may be mentioned carbon black, graphite, coke powder and the like; the type and amount of carbon powder used may be appropriately selected depending on the resistance value and shape of the heating element to be produced, and any one or combination of two or more may be used, although graphite is particularly preferred to allow easier control of the shape.